In previous railroad grade crossing protection systems for detecting and/or monitoring approaching trains, it was generally common practice to employ one of two techniques for obtaining the track parameters which controlled the highway warning apparatus. The first method employed an arrangement wherein a constant amplitude alternating current was transmitted and introduced into the track rails at the crossing location. The location of an approaching train and an indication of its velocity was obtained by measuring the amplitude of the voltage developed in the track circuit and the rate of change of the developed voltage. The second method involves incorporating the track circuit in a frequency determining network of an electronic oscillator. Thus, the reactance and resistance of the track circuit establish the frequency of oscillation. The frequency is constantly monitored and any frequency change is automatically adjusted to maintain the frequency constant. Thus, the rate of the frequency adjustments is detected to provide an indication of any train motion in the track circuit. It has been found that the existing motion monitors or detectors are possessed of several inherent shortcomings. For example, the monitoring apparatus is directly wire-connected to the track rails so that loading of the track circuit results in the desensitization of the motion detection. In the case of a center-fed track circuit, the reflected track impedance is equivalent to the product of the impedances of the two approach zones divided by their sum. Thus, at low ballast conditions, the measurement of the change in impedance of one approach zone is relatively difficult due to the loading of the other approach zone. Further, the sensitivity is adversely effected by the circuit being loaded by the lumped ballast resistance which occurs at the crossing area due to the accumulation or buildup of mud, salt, cinders, and other foreign substances which takes place during the winter season. In addition, the prior art motion detectors are susceptible to interference and inaccurate train prediction in coded track territory which causes alternate loading and unloading of the track circuit. Thus, the impedance presented to a previous continuous monitoring apparatus is in a state of constant flux so that the distance and velocity of an approaching train, which are functions of the value and rate of change of the track impedance, are unpredictable.